1. Field of the Invention
The present invention relates to a hydrodynamic bearing device, a motor, and a disk driving apparatus, and more particularly relates to a hydrodynamic bearing device that utilizes the dynamic pressure of a fluid and is used, for example, in a motor for rotationally driving a disk-shaped recording medium, and to a motor and a disk driving apparatus in which this hydrodynamic bearing device is used.
2. Background Information
Recent years have seen the data transfer rate rise along with memory capacity in disk driving apparatuses in which a disk-shaped recording medium such as a magnetic disk, optical disk, or opto-magnetic disk is rotationally driven. Accordingly, a hydrodynamic bearing device with which a shaft that is rotationally driven at high speed can be maintained at high precision is used for the bearing devices of motors used in this kind of disk driving apparatus.
With a typical hydrodynamic bearing device, the space between the outer peripheral face of a shaft and the inner peripheral face of a holding component for holding this shaft is filled with a lubricating oil that serves as a working fluid, dynamic pressure is generated by a herringbone groove formed in the outer peripheral face of the shaft or the inner peripheral face of the holding component, and this forms a radial bearing that supports the load of the rotating body in the radial direction during rotation. Lubricating oil also fills the space between the holding component and a disk-shaped thrust plate affixed to the end of the shaft, dynamic pressure is generated by a spiral groove formed in any opposing faces of the thrust plate and the holding component, and this forms a thrust bearing that supports the load of the rotating body in the axial direction during rotation.
A hydrodynamic bearing device that has already been proposed will be described in specific terms through reference to FIG. 17. FIG. 17 is a cross section illustrating the structure of a motor for a disk driving apparatus in which the hydrodynamic bearing device disclosed in Japanese Laid-Open Patent Application 2000-350408 is used.
In FIG. 17, the proposed disk driving apparatus motor is equipped with a rotor hub 2 on which a magnetic disk or other such disk-shaped recording medium 1 (hereinafter referred to as “disk 1”) is mounted, a shaft 3 provided passing through the rotor hub 2 in the axial direction, a base 4 that fixes the shaft 3 and supports a core 5 of a motor stator, and a rotor magnet 6 that is disposed across from the core 5 and fixed to the rotor hub 2. A herringbone groove is formed in the outer peripheral face of the shaft 3 or the inner peripheral face of the rotor hub 2, and a spiral groove is formed on the lower face of the rotor hub 2 or the upper face of the base 4. A lubricating oil 7 fills the tiny space between the opposing faces of the rotor hub 2 and the shaft 3, and this forms a radial bearing. The lubricating oil 7 also fills the tiny space between the opposing faces of the rotor hub 2 and the base 4, and this forms a thrust bearing.
As shown in FIG. 17, a cutout 3a is formed in the upper end of the shaft 3, and an annular plate 8 protruding in the radial direction from the outer peripheral face of the shaft 3 is affixed to this cutout 3a. This plate 8 is disposed corresponding to a stepped portion 2a of the rotor hub 2, and has the function of keeping the rotor hub 2 from coming off.
With the proposed disk driving apparatus motor that makes use of a hydrodynamic bearing device constituted as above, when the drive component constituted by the core 5 and the rotor magnet 6 is excited, this causes the rotor hub 2 on which the disk 1 is mounted to rotate, and the bearing functions of the radial bearing and thrust bearing are realized. Specifically, when electrical power is sent to the core 5, the rotor hub 2 rotates with respect to the shaft 3 and the base 4, in the thrust bearing the lubricating oil 7 between the lower face of the hub 2 and the upper face of the base 4 generates dynamic pressure, which supports the load in the thrust direction, and in the radial bearing the lubricating oil 7 between the outer peripheral face of the shaft 3 and the inner peripheral face of the hub 2 generates dynamic pressure, which supports the load in the radial direction.
The hydrodynamic bearing device shown in FIG. 18 is also known. FIG. 18 is a cross section illustrating the structure of a motor for a disk driving apparatus in which the hydrodynamic bearing device disclosed in U.S. Pat. No. 5,558,445 is used.
The spindle assembly 10 shown in FIG. 18 is primarily made up of a base 12, a shaft 14 fixed to the base 12, an annular upper thrust bearing plate 28 fixed to the upper end side of the shaft 14, an annular lower thrust bearing plate 30 fixed to the lower end side of the shaft 14, a shaft housing (sleeve) 16 mounted rotatably around the outside of the shaft 14, a spindle hub 18 fixed to the outer peripheral side of the shaft housing 16, an annular magnet 31 and a flux circulation ring 33 that are mounted on the spindle hub 18, and a stator assembly 29 fixed to the base 12 across from the annular magnet 31 in the radial direction. With this spindle assembly 10, the shaft 14 and the shaft housing 16 are across from each other in the radial direction, forming radial bearings 34 and 36. Also, the upper and lower thrust plates are across from each other in the thrust direction, forming thrust bearings 40 and 42.
Electronic devices provided with disk driving apparatuses today are tending to be smaller, lighter, and thinner, and this trend is particularly pronounced in portable electronic devices. As a result, reducing the size, weight, and thickness of motors for disk driving apparatuses used in these electronic devices is an important goal in this field. Therefore, reducing the size, weight, and thickness is also something to be achieved in hydrodynamic bearing devices used as the bearing devices in these motors for disk driving apparatuses.
With the proposed disk driving apparatus motor featuring a hydrodynamic bearing device shown in FIG. 17, the base 4, which is thick because it is used to fix the shaft 3, is provided on the lower side of the rotor hub 2 on which the disk 1 is mounted. The annular plate 8 fixed to the shaft 3 must be attached to keep the rotor hub 2 from coming off. Thus, the proposed disk driving apparatus motor makes use of a base 3 that is thick so that the lower end of the shaft 3 can be affixed in an opening in the base 4, and enough space has to be ensured so that the retaining plate can be installed. These requirements, however, are contrary to the goal of reducing the size, weight, and thickness of a hydrodynamic bearing device and a disk driving apparatus motor. Furthermore, with the disk driving apparatus motor shown in FIG. 17, the bearing can be completed and its characteristics evaluated only after the base 4, the shaft 3, the hub 2, and the plate 8 have been assembled and oil added, so the oil filling equipment is expected to end up being large and the yield low.
With the disk driving apparatus motor featuring a hydrodynamic bearing device shown in FIG. 18, since the annular upper thrust bearing plate 28 and the annular lower thrust bearing plate 30 are disposed in the axial direction, and a radial bearing is provided between these thrust bearing plates, it is difficult to keep the structure thin while still ensuring adequate radial bearing performance.
It is an object of the present invention to provide a hydrodynamic bearing device with high reliability, with which reductions in size, weight, and thickness can be achieved, as well as a motor and a disk driving apparatus that make use of this hydrodynamic bearing device. It is a further object to effectively discharge bubbles and smoothly circulate the lubricating fluid in a bearing structure proposed for the sake of reducing size.